The thermodynamic properties of calcium-calcium halide fluxes were measured. The removal behavior of tramp elements from molten metals were discussed based on the observed results.The activities of calcium in the CaO_<satd>. -Ca-MF_2 (M : Ca, Ba, Mg) and CaO_<satd>. -Ca-NaF systems were measured as a function of calcium composition between 1373 and 1573K by equilibrating these fluxes with solid zirconium plate in a CaO crucible.The CaO_<satd>. -Ca-CaF_2 is saturated with CaF_2 at X_<Ca>/ [X_<Ca>+X_<CaF2>] =0.80 and the activity of Ca is 0.77. The activity of Ca in the CaO_<satd>. -Ca-CaCl_2 flux is smaller than that in the CaO_<satd>. -Ca-CaF_2 flux. The CaO_<satd>. -Ca-BaF_2 is saturated with BaF_2 at X_<Ca>/ [X_<Ca>+X_<BaF2>] =0.66. The activity of Ca in the CaO_<satd>. -Ca-MgF_2 is 0.14 at X_<Ca>/ [X_<Ca>+X_<MgF2>] =0.50. The CaO_<satd>. -Ca-SrF_2 is saturated with SrF_2 at X_<Ca>/ [X_<Ca>+X_<BaF2>] =0.15 and the activity of Ca is 0.04. The activity of Ca in the CaO_<satd>. -Ca-NaF is 0.48 at X_<Ca>/ [X_<Ca>+X_<NaF>] =0.60.The contents of tramp elements such as antimony, arsenic, bismuth, lead and tin of molten carbon saturated iron were estimated by using the observed data of calcium activity in these fluxes. Content of each tramp element was estimated to be smaller than 10^<-4> mass%. Therefore, the removal of tramp elements such as tin, arsenic, antimony, bismuth and lead from molten rare metals by using calcium-metal fluoride fluxes may be possible.これらの熱力学データから、カルシウム系フラックスを用いた場合の、溶鉄中のトランプエレメント(アンチモン、砒素、ビスマス、鉛、すず)の濃度を見積もり、いずれの元素も、10^<-4>mass%以下に除去されることがわかった。このことから、本系フラックスによるレアメタルの高純度化も可能と考えられる。